DESCRIPTION (Applicant's Abstract): Opiates are powerful analgesic drugs. Unfortunately, their chronic use often results in addiction and tolerance, liabilities that seriously limit their clinical usefulness. Changes at the cellular and opiate receptor level have proven to be inadequate explanations of tolerance and dependcence. An exciting alternative hypothesis is that "anti-opioid" systems, neurotransmitters or neuropeptides that functionally oppose the effects of opiate receptor activation, contribute to tolerance and dependence as well as to variations in analgesic potency of opioids in some pathological pain states. We have recently demonstrated the orphanin FQ (OFQ, also referred to as nociceptin) is the endogenous ligand for the opioid-like G protein-coupled receptor LC132 and that it reverses morphine-induced analgesia, hypothermia and Straub tail. These anti-opioid actions of OFQ/N together with its close evolutionary kinship to endogenous opioid peptides suggesty that the OFQ/LC132 neurotransmitter system may play an important role in the homeostasis of opioid mechanisms. We propose three complimentary Specific Aims taht are designed to clarify the interactions between the opiate and the OFQ/LKC132 neuropeptide transmitter systems. The experiments included in Specific Aim (1) are designed to extend our knowledge about the OFQ/N neuropeptide system at the molecular level. Specifically we propose to quantitate changes in OFQ/N and its receptor that may occur in opiate-tolerant animals. We also outline experiments designed to characterize two new forms of the receptor that we discovered since ourr last subsmission. The last set of experiments included in Specific Aim described our ongoing efforts to produce a strain of mouse that lacks the OFQ/N receptor. These mice should prove to be an important animals model in which to evaluate the involvement of the OFQ/N receptor system in mediating opiate tolerance, dependence and reward. In Specific Aim (2) we have proposed studies that continue our characterization of OFQ/N's electrophysiological effects on well-defined neurons in the ventral tegmental area. This area of the is known to be important in reward behavior and its neurons are known to be altered in animals repeatedly exposeds to morphine. The experiments proposed in specific aim (3) involve the identification of some of the neuroanatomical substrates that mediate OFQ/N's ati-opioid activity. These microinjection studies will provide us with an elegant means by which to determine whetheer OFQ/N is able to produce a tolerance-like effect in drug-naive rats. These studies will advance our understanding OFQ/N's involvement in opioid analgesia as well as tolerance, and could ultmately lead to improved treatment of pain and addiction.